Method and Apparatus for Fluid Separation

ABSTRACT

A method and apparatus are disclosed for separating a multiphase fluid stream that includes a heavier fluid component and a lighter fluid component. The fluid flows along a first helical flowpath with a first pitch. The first helical flowpath is sufficiently long to establish a stabilised rotating fluid flow pattern for the stream. The uniform rotating fluid also flows along a second helical flowpath, the second helical flowpath having a second pitch greater than the first pitch. The lighter fluid is removed from a radially inner region of the second helical flowpath. The method and apparatus are particularly suitable for the separation of oil droplets from water, especially from water for reinjection into a subterranean formation as part of an oil and gas production operation. The method and apparatus are conveniently applied on a modular basis.

CROSS REFERENCE TO RELATED APPLICATIONS

This application is a divisional of U.S. patent application Ser. No.12/280,664 filed Oct. 15, 2008 entitled “Method and Apparatus for FluidSeparation,” which is a National Phase under 35 U.S.C. §371 ofInternational Application No. PCT/GB2007/000601, filed Feb. 22, 2007,entitled “Method and Apparatus for Fluid Separation,” which claims thebenefit of United Kingdom Patent Application No. 0603811.1, filed Feb.25, 2006, entitled “Method and Apparatus for Fluid Separation,” whichis/are incorporated herein by reference in its/their entirety for allpurposes.

The present invention relates to a method and apparatus for theseparation of multiphase fluid streams. The method and apparatus findparticular application in the separation of multiphase liquid streams,especially the separation of hydrocarbon liquids from water. The methodand apparatus are particularly suitable for the purification of waterproduced from subterranean oil and gas wells.

Hydrocarbons produced from a subterranean well, such as oil and gas, areaccompanied by quantities of other materials, including water. In somecases, the volume of water produced from a well can be significant. Inmany situations, the produced water is disposed of by being reinjectedunderground, either into the same well from which it is produced, or aneighbouring well. The requirements for the purity of the water beingreinjected in such a manner are strict. In particular, it is importantthat the solids content of the water is low and that the water containsa minimal amount of entrained oil. In general, it is required that thewater for reinjection contains less than 400 ppm oil and less than 2 ppmsand. Still lower values may be required in certain locations. Theserequirements must be met in order to prevent the well from becomingplugged and to meet legal requirements pertaining to water reinjection.

Conventional techniques for cleaning and purifying the water producedfrom wells ready for reinjection rely upon the use of settling tanks,into which the mixed fluid stream is fed and separation of the lighteroil fraction from the denser water fraction takes place under the actionof gravity. The very small size of the oil droplets entrained in thewater requires a long residence time in a settling vessel in order forgravity separation to be effective. This in turn requires the vessel tobe of a large volume. Such a large vessel would be costly to manufactureand install close to the wellhead in a subsea location. Indeed, it maynot be feasible to manufacture a vessel with sufficient burst orcollapse strength to operate under the hydrostatic pressures encounteredat many deep water wellhead locations. Accordingly, settling vessels aregenerally located at the surface, on a fixed or floating platform. Thisnecessitates providing suitable pipework to transfer the water from theseabed to the surface and return the polished water to the seabed forreinjection. In addition, due to their size, the settling vessels occupya large volume of space on the surface structure, space which is veryoften at a premium. A further problem is that the separation efficiencyof the settling tanks is generally low and only approaches acceptablelevels after excessively long residence times for the water in the tank.This in turn increases the volume of the tank further. Accordingly,there is a need for an improved system for purifying produced water torender it suitable for reinjection.

An alternative technique for removing oil from water is the use of ahydrocyclone, often referred to in the art as “de-oilers.” These devicesare advantageous in having a high separation efficiency compared withgravity separation, being compact and an absence of moving parts. Onearrangement of hydrocyclones is a tiered or series assembly. The firstcyclone in the series is a bulk oil-water cyclone (BOW), in which theoil concentration of the feed is reduced from as much as 50% to 15%, byvolume. The water is then passed to a pre-de-oiler cyclone (PDC), inwhich the concentration of oil is further reduced to about 0.2%. Thefinal stage of cyclone separation is the de-oiler. A problem exists inthat the hydrocyclones are effective as de-oilers only at low liquidflow rates. For example, a typical maximum throughput is of the order of1200 barrels per day (bpd). However, it is necessary for the de-oilerassembly to operate over a much wider range of flowrates, for example upto 40,000 bpd. Known hydrocyclone technology does not allow the cyclonede-oiler to operate over such a wide range of flowrates and achieve aconsistently high separation efficiency.

Accordingly, there is a need for an improved separation system that isable to achieve a high separation efficiency over a wide range of fluidflowrates. It would also be very advantageous if the system was able tobe located at the wellhead at a subsea location, where the fluid leavingthe well has the highest temperature and the least viscosity.

EP 1 352 679 discloses a separator for separating a multiphase flow, theseparator comprising an inlet for a multiphase fluid, a plurality ofoutlets, with at least one outlet being provided for each separatedphase, and a main annular tubular bore. The separator operates toseparate lighter and heavier components by causing the fluid to flow ina rotational path. While this separator is particularly effective inseparating multiple fluid phases, such as gas, oil and water, it cannotguarantee the high separation efficiency required in order to purifyproduced water sufficiently to allow for reinjection. In particular,sufficient oil droplets remain in the water product of this separator toprevent the water from being reinjected directly into an undergroundformation. In order to further purify the water, it is necessary toprovide a system that is low in shear, such that the remaining minutedroplets of oil are not emulsified with the water fraction, as suchemulsification would make further separation very difficult, if notimpossible within a reasonable time frame.

GB 2 374 028 A discloses a separator for oil and water mixturesemploying a vortex separator to remove the bulk of the oil from thewater. The resultant oil/water mixture is passed through a stack oftilted plates to remove further oil droplets from the water. The systemof GB 2 374 028, while capable of separating oil from water, is notcapable of providing sufficient separation for the water to bereinjected into an underground formation.

Accordingly, there is a need for an improved separation technique toenable multiphase fluid streams to be separated, in particular streamsof water and oil, such that the water may be sufficiently cleaned of oilto allow for reinjection into an underground formation.

According to the present invention, there is provided in a first aspect,a method for separating a multiphase fluid stream comprising a heavierfluid component and a lighter fluid component, the method comprisingcausing the fluid to flow along a helical flowpath in which the criticalReynolds number of the fluid flow is elevated, the fluid stream flowingat a Reynolds number below the elevated critical number, the fluidstream flowing at a sufficient velocity to cause the fluid phases toseparate.

The first aspect of the present invention employs the phenomenum that afluid forced to flow in a confined conduit, such as between two platesor the like, exhibits different flow regimes to the same fluid flowingin an open conduit or a pipe. In particular, the forced fluid flowexhibits a significantly increased critical Reynolds number, that is theReynolds number at which turbulent flow begins. This in turn allows thefluid velocity to be significantly increased, while still maintaining anon-turbulent flow regime. References to an “elevated critical Reynoldsnumber” are to be construed accordingly.

By forming the helical flowpath so as to give rise to an elevatedcritical Reynolds number, the rotational velocity of the fluid can besignificantly increased, enhancing the separation of the differentphases. Preferably, the critical Reynolds number is greater than 10,000,more preferably greater than 100,000.

In a second aspect, the present invention provides a method forseparating a multiphase fluid stream comprising a heavier fluidcomponent and a lighter fluid component, the method comprising causingthe fluid to flow along a helical flowpath extending around a centralconduit, the fluid flowing at a sufficient velocity to cause the lighterfluid component to move to the inner region of the helical flowpath; andcollecting the lighter fluid component in the central conduit.

Preferably, the method of this aspect utilises the aforementionedprinciple of elevating the Reynolds number of the fluid stream. Thecritical Reynolds number of the fluid flow is elevated, while the fluidstream is maintained flowing at a Reynolds number below the elevatedcritical number.

In a further aspect, the present invention provides a method forseparating a multiphase fluid stream comprising a heavier fluidcomponent and a lighter fluid component, the method comprising: causingthe fluid to be forced along a first helical flowpath, the first helicalflowpath having a first pitch, the first helical flowpath beingsufficiently long to establish a stabilised rotating fluid flow patternfor the stream; causing the uniform rotating fluid to flow along asecond helical flowpath, the second helical flowpath having a secondpitch, wherein the second pitch is greater than the first pitch; andremoving the lighter fluid from a radially inner region of the secondhelical flowpath.

The method of the present invention is suitable for the separation ofany multiphase fluid stream, including streams comprising one or moreliquid phases and one or more gas phases. The method is particularlysuitable for the separation of multiphase liquid-liquid streams. Themethod is particularly advantageous in its efficiency at separatingliquid hydrocarbons, especially crude oil, from aqueous streams. Oneapplication of the method is the separation of crude oil from waterproduced from a subterranean well, prior to the reinjection of theproduced water into an underground formation.

The method is particularly suitable for separating a minor fraction of adispersed first fluid from the bulk or continuous phase of a secondfluid. Preferably, the lighter fluid fraction is the dispersed phase.

In the first step of the separation method, the incoming fluid isdivided into manageable portions allowing a suitable flow rate to beachieved. The or each portion is preferably first caused to tangentiallyenter a separator region, thereby imparting sufficient rotationalvelocity on the fluid to cause the phases to begin to congregate. Inthis cylinder separation, the phases in the stream congregate andcoalesce, thereby allowing the dispersed phase to form as largerdroplets.

The fluid stream is then caused to rotate in a compact helix underpressure, so that the fluid is subjected to a high centrifugal force,allowing the fluid to form a stable rotating flow pattern. In order toavoid the different fluid phases from becoming further mixed, inparticular emulsified, the fluid stream is stabilised into a flow regimethat is below the critical Reynolds number (that is the Reynolds numberabove which the flow regime is turbulent). The critical Reynolds numberwill depend upon such factors as the viscosity and density of the fluidstream, the velocity of the fluid stream and the dimensions of theconduit through which the stream is passing. Preferably, the fluid isstabilised in a transient flow regime, thus keeping the droplets of thedispersed fluid phase active. In the present invention, the compacthelix is arranged such that the Reynolds number can be significantlyhigher than the usual critical number, while still having the fluid in alaminar or transitional flow regime. Such an effect, generated forexample when a fluid is caused to flow between two facing plates, isknown in the art. This effect is employed in the present invention, inorder to allow a high rotational fluid velocity to be achieved, whilemaintaining the fluid in a non-turbulent flow regime. In this way, theseparation of the various phases due to the centrifugal forces isenhanced.

The length of the first helical flowpath should be of sufficient lengthto allow the fluid flow to centrifugally establish and stabilise in therequired flow regime, most preferably a transient flow regime. Thenature of the fluid stream, its components and the flow regime of thefluid being processed in the method will determine the length of thefirst helical flowpath. If the required flow regime can be establishedquickly, the first helical flowpath will be correspondingly short.

Once a stabilised flow regime has been established, the fluid stream iscaused to flow along a first helical flowpath. In this step, the fluidis acted upon by a centrifugal force to create a multiple gravity force,as a result of being forced to flow along the helical path, the effectof which is to cause the heavier fluid to be forced to the outercylindrical wall and the lighter fraction or fractions to migrate to theinner region of the helix. The helical flowpath has a first pitch. It ispreferable that the pitch of the helical flowpath remains constantthroughout the length of the first helical flowpath, as the fluid flowis being pressurised through the helix plates.

Thereafter, the fluid stream is led into a second helical flowpath, fromwhich the lighter fluid phase is recovered. The second helical flowpathhas a second pitch that is greater than the pitch of the first helicalflowpath. The second pitch may be constant throughout the length of thesecond helical flowpath. However, in order to reduce friction losses inthe fluid stream as a result of back-pressure, it is preferred that thecross-section area of the second helical flowpath increases along itslength. This is most conveniently achieved by having the pitch of thesecond helical flowpath increase along its length. The pitch mayincrease step-wise or gradually. In one preferred embodiment, the pitchof the second helical flowpath increases continuously along the lengthof the second helical flowpath. In a preferred arrangement, the pitchincreases by up to 5% for each turn of the fluid flowpath around thelongitudinal axis of the flowpath, more preferably up to 3%, especiallyabout 1% for each turn. In this way, a flow regime is maintained thatallows the lighter fluid fractions to migrate to the inner region of thehelical flowpath, from where they are removed.

The second helical flowpath should be long enough to allow the lighterfluid phases to be collected and removed from the fluid stream. Smalldroplets of the lighter fluid may remain in the bulk heavier fluidphase. If so, and the desired or required level of fluid purity has notbeen achieved, further processing stages may be employed, as follows.

Should further separation and purification be required, the method maycomprise further steps, in which the rotational velocity of the fluidstream is increased so as to generate a central vortex of lighter fluidfractions, from which light fluid may be withdrawn. The increase inrotational velocity may be achieved using a third helical flowpath,along which the cross-sectional area of the flowpath is adjusted so asto cause the increase in fluid velocity required to generate the vortex.In one preferred arrangement, the pitch of the third helical flowpathincreases in the direction of flow. The increase in pitch may bestepwise or continuous. Preferably, the pitch of the third helicalflowpath increases along substantially its entire length. In order togenerate the required increase in fluid velocity, the helical flowpathnarrows in width in the radial direction, as the pitch increases. Theincrease in fluid velocity is preferably controlled such that thecritical Reynolds number of the fluid flow is not exceeded. Thecross-sectional area of the third helical flowpath is such thatexcessive friction losses and back-pressure are avoided.

After the increase in the rotational velocity, the fluid is ejected fromthe third helical flowpath in the form of a rotating annulus wall offluid, which contains a rotating core of fluid. Within the rotating coreof fluid, a separation vortex is established. In this stage, theremaining lighter fluid is caused to migrate towards and into thevortex, with the heavier fluid circulating in the annular regionextending around the established vortex. At this point, a helicalflowpath need not be provided and the aforementioned flow regimes andthe vortex can be established in an open conduit, such as a tube orpipe. In this way, the vortex is established at the exit of the secondhelical flowpath.

In many cases, the vortex induced in this way is relatively short, incomparison with the length of the surrounding conduit. In such cases ofa short vortex, the stability of the vortex may be reduced, leaving thevortex susceptible to minor changes in the flowrate of the fluid.Accordingly, it is preferred to provide a means for stabilising thevortex. In one preferred embodiment, the vortex is formed beneath aconduit for removing the lighter fluid that has migrated to andcollected in the vortex. A preferred means for capturing and stabilisingthe vortex is a guide cone and guide conduit of suitable dimensionsdisposed within the conduit in the region of its opening into thevortex. In this way, the vortex is stabilised both within the entryregion of the conduit and in the bulk fluid.

The fluid stream leaving the vortex separation region will containlittle or no lighter components and will consist mainly of the heavierfluid components. Should some lighter components remain, furtherseparation steps may be carried out as follows.

In a preferred embodiment of the present invention, the method furthercomprises introducing the fluid stream into a fluid-fluid settlingregion, in which the lighter fluid components are separated from theheavier fluid components under the action of gravity. The velocity ofthe fluid stream in the fluid-fluid settling region is significantlylower than in the previous separation regions or zones. In particular,the velocity is such that the Reynolds number of the fluid stream iswell below the critical Reynolds number, most preferably in the laminarflow regime.

Preferably, the fluid stream is caused to rotate in the fluid-fluidsettling region. This is most advantageously achieved by having therotation imparted to the fluid stream upon exiting the vortex separationregion. While the major separation effect in this region is the gravityseparation, the rotational flow regime will cause the lighter fluidcomponents to concentrate in the central or innermost zone of theregion, allowing for an easier removal and improved separation.

To assist with the separation of any remaining lighter fluid components,the method preferably comprises centralising the rotational flow of thefluid stream within the fluid-fluid settling region. This is preferablyachieved in a manner in which the cross-sectional area of the fluid flowpath in the fluid-fluid settling region is reduced in the direction offlow.

In one preferred arrangement, fluid richer in the lighter fluidcomponent is removed from the lower central region of the fluid-fluidsettling region and passed to the upper central region of thefluid-fluid settling region. To prevent remixing at the exiting vortexregion, an axial cowling may be provided to enable the lighter fluiddroplets to move to the lighter fluid region unhindered by the rotatingbulk phase. In this way the separation of the lighter and heaviercomponents is enhanced. In particular, the lighter components are movedto the upper portion of the settling region, which is already relativelyrich in the lighter components, with the heavier components thustransported returning to the lower portion under the effects of gravity.

Monitoring, fluid sampling and fluid injection ports and lines may beprovided that terminate at appropriate locations within the system. Forexample, lines may be provided to inject pressurised gas or agas-fluidised liquid to cause upwardly moving bubbles to ascend throughthe heavier fluid phase. This would assist with the predominantlygravity separation effect. In particular light phases, such as oil andother hydrocarbons may adhere to the surface of the gas bubbles and thenbe carried at a faster rate to the lighter fluid regions, as is commonlyemployed in floatation separation processes.

If desired, the separation of the lighter fluid component from theheavier fluid components may be enhanced by the addition of additivesactive in inducing droplet coalescence of the lighter fluid component.Suitable additives are well known in the art and are commerciallyavailable. Scale inhibitors may be applied to prevent the formation ofscale, in particular in locations where there is a significant pressuredrop in the fluid stream, such as in the inlet to the system of thepresent invention. Demulsifiers may be added upstream of the first stageof separation, in order to enhance the separation of the fluid phases.Corrosion inhibitors may be required in the system of present invention,in particular downstream of the helical separation assemblies.Coalescers may be introduced as required in the system in order topromote the aggregation of fluid phases. Wax inhibitors may be requiredwhen oil is present as one of the fluid phases, in order to prevent thecrystallisation of high molecular weight wax compounds in the regions ofhigh oil concentration. Other additives that may be employed includefriction reducers, hydrate inhibitors and biocides.

The remaining fluid in the process will consist almost entirely of theheavier component or components. These are preferably passed to a fluidremoval region, in which a fluid stream consisting essentially of theheavier fluid component is removed. The fluid stream is preferablycaused to rotate in the fluid removal zone, the said fluid stream beingremoved from the central region of the fluid removal zone. In this way,any heavy components, such as sediment or the like, may be collectedunder the action of gravity and removed from the system, for example ona batch wise basis as sufficient sediment collects in a suitablereceptacle.

A particular advantage of the method of the present invention is that itmay be applied on a modular basis. In this way, a wide range ofoperating fluid flowrates may be accommodated and a separation processprovided that may be applied for extended periods of time withsignificant variations in the fluid throughput. This is of particularadvantage in the application of the method to separation in remotelocations, especially subsea wellhead operations.

Accordingly, the present invention also provides a method for separatinga multiphase fluid stream comprising a heavier fluid component and alighter fluid component, the volume flowrate of the multiphase fluidstream being subject to variation over time, the method comprisingproviding a plurality of separation assemblies for carrying out themethod steps of: allowing a stream of controlled flowrate to enter adedicated separation assembly; establishing a stabilised rotating fluidflow pattern for the stream; causing the stabilised rotating fluid to beforced along a first helical flowpath, the first helical flowpath havinga first pitch; causing the uniform rotating fluid to flow along a secondhelical flowpath, the second helical flowpath having a second pitch,wherein the second pitch is greater than the first pitch; and removingthe lighter fluid from a radially inner region of the second helicalflowpath; wherein the assemblies are operable to accommodate differentfluid flowrates; and selecting one or more separation assemblies forcarrying out the method steps according to the volume flowrate of themultiphase fluid stream.

The method steps carried out in each separation assembly may have any ofthe preferred or specific features hereinbefore described.

In one embodiment, the modular separation method further comprisesproviding a finishing assembly for carrying out the fluid-fluid settlingsteps described hereinbefore, wherein each of the plurality ofseparation assemblies is connected at its outlet to the finishingassembly.

In addition to the aforementioned method aspects of the presentinvention, there is also provided corresponding apparatus aspects. Thus,in a first aspect, the present invention provides an apparatus forseparating a multiphase fluid stream comprising a heavier fluidcomponent and a lighter fluid component, the apparatus comprising ahelical flowpath having a fluid inlet, a first outlet for a heavierfluid component and a second outlet for a lighter fluid component, thehelical flowpath being formed such that the critical Reynolds number ofthe fluid stream flowing along the helical flowpath is elevated.

Preferably, the second outlet for the lighter fluid component isdisposed axially centrally of the helical path, in particular openinginto an axially central lighter fluid conduit.

The helical flowpath may be arranged to provide the elevated criticalReynolds number hereinbefore described. In particular, this may beachieved by adjusting the internal dimensions of the helical flowpathaccording to the properties of the fluid stream to be processed.

In a further aspect, the present invention provides an apparatus forseparating a multiphase fluid stream comprising a heavier fluidcomponent and a lighter fluid component, the apparatus comprising: meansfor selecting a stream of fluid of predetermined flowrate; a firstconduit for establishing a stabilised rotating fluid flow pattern forthe fluid stream having a first helical flowpath, the first helicalflowpath having a first pitch; a second conduit having second helicalflowpath, the second helical flowpath having a second pitch, wherein thesecond pitch is greater than the first pitch; and means for removing thelighter fluid from a radially inner region of the second helicalflowpath.

The apparatus comprises a first conduit having a helical passagetherethrough, through which fluid may be caused to flow in a helicalflowpath. In order to avoid the different fluid phases from becomingfurther mixed, in particular emulsified, the first conduit is preferablyformed such that the fluid stream is stabilised into a flow regime thatis below the critical Reynolds number (that is the Reynolds number abovewhich the flow regime is turbulent). The critical Reynolds number willdepend upon such factors as the viscosity and density of the fluidstream, the velocity of the fluid stream and the dimensions of theconduit through which the stream is passing. Accordingly, the specificshape, dimensions and length of the first conduit will be determined bythe properties of the feed stream be processed. Preferably, the conduitis of a size such that the fluid is stabilised in a transient flowregime, thus keeping the droplets of the dispersed fluid phase active, Aparticularly preferred arrangement is for the first conduit to comprisea regular tube having an internal helix in order to provide the helicalflowpath.

The length of the first helical flowpath within the first conduit shouldbe sufficient to allow the fluid flow to stabilise in the required flowregime, most preferably a transient flow regime. The nature of the fluidstream, its components and the flow regime of the fluid being processedin the method will determine the length of the first helical flowpath.The first helical flowpath is of sufficient length to allow the time forthe centrifugal separation of the lighter fluid phase from the heavierfluid bulk phase. If the required flow regime can be establishedquickly, the first helical flowpath will be correspondingly short.

Preferably, the pitch of the first helical flowpath remains constantthroughout its length.

The apparatus is preferably provided with a feed conduit, into which themultiphase fluid stream to be separated is forced, prior to entering thefirst conduit. The apparatus preferably comprises a means forestablishing a rotating fluid flow in the feed conduit. Preferably, thefeed conduit comprises a tangential opening, through which the feedstream is forced, the arrangement of the tangential opening causing thefluid to rotate as it passes along the feed conduit and be subjected tohigh centrifugal forces providing an initial region of separation of thephases before the fluid enters the first conduit.

The apparatus further comprises a second conduit, in which separation ofthe different phases of the fluid stream takes place. The second conduitalso comprises a helical flowpath extending therein. In a preferredarrangement, the second conduit comprises a tube having a helixextending longitudinally therein to provide a second helical flowpath.Preferably, the pitch of the second helical flowpath increases in thedirection of flow along the second flowpath. The increase in the pitchmay be in a stepwise or a continuous manner. In a preferred arrangement,the pitch of the second helical flowpath is increased alongsubstantially the entire length of the second helical flowpath withinthe conduit. The pitch may increase up to 5% for each turn of the secondhelical flowpath around the longitudinal axis of the flowpath,preferably up to 3%, more preferably about 1% for each turn.

To separate the lighter fluid phase from the heavier fluid phases, ameans for removing the lighter fluid phase is provided within the secondconduit. Preferably, the means for removing the lighter fluid comprisesa collection conduit extending coaxially within the second conduit, thehelix extending within the annulus around the collection conduit.

The feed conduit, first and second conduits may comprise separatecomponents of the apparatus. However, in a most convenient arrangement,the feed conduit, first and second conduits are adjacent portions of asingle tube, a first helix being provided in an upstream portion of thetube to provide the first helical flowpath and a second helix beingprovided in a downstream portion of the tube to provide the secondhelical flowpath. In such an arrangement, the means for removing thelighter fluid may comprise a collection conduit extending coaxiallywithin the single tube, the collection conduit having openings in theportion extending within the downstream or second portion for thecollection of fluid.

In many circumstances, the provision of the apparatus with first andsecond conduits, optionally with a feed conduit, will result in anacceptable centrifugal separation of the lighter and heavier fluidphases. However, should further separation be required, the apparatusmay comprise one or more of the following components.

Should further separation be required or desired, a preferred techniqueis the use of a vortex separation action, which subjects the remainingflow to very high centrifugal forces. Accordingly, in such a case, theapparatus may further comprise a conduit for retaining a vortex, thesaid conduit being arranged to receive fluid leaving the second helicalflowpath. In order to provide the optimum vortex for fluid-fluidseparation, the rotational velocity of the fluid stream must be suitablyhigh. Accordingly, if required, the apparatus may further comprise ameans for increasing the rotational velocity of the fluid disposedbetween the outlet of the second helical flowpath and the inlet to theconduit for retaining a vortex. Suitable means for increasing therotational velocity of the fluid is a third helical flowpath. In orderto provide the necessary velocity increase, the cross-sectional area ofthe third helical flowpath decreases along the length of the flowpath.The decrease in the cross-sectional area may occur in a continuous or astep-wise manner. Preferably, the decrease in the cross-sectional areaoccurs along substantially the entire length of the third helicalflowpath. The third helical flowpath is most conveniently formed withina downstream portion of the same conduit containing the first and secondhelical flowpaths.

The vortex may be allowed to form within a substantially empty conduit,such as an empty downstream portion of the conduit containing the first,second and, if present, the third helical flowpaths. In some processregimes, it may be necessary to provide a means for stabilising thevortex. In one preferred arrangement, the apparatus further comprises aconduit for collecting the lighter fluid component from the vortex, themeans for stabilising the vortex is provided by a tapered portion in theregion of the opening of the said conduit.

The apparatus described hereinbefore may conveniently be housed within asingle conduit or tube, as already mentioned. Further separation may beprovided by way of an essentially gravity separation process.Accordingly, the apparatus may further comprise a vessel for receivingthe fluid stream, the vessel having a volume sufficient to reduce theReynolds number of the fluid stream flow such that fluid entering thevessel may be subjected to gravity separation. In such an arrangement,the single tube or conduit as hereinbefore described may convenientlyextend within the vessel. In one preferred arrangement, the apparatusmay be modular in design, as described hereinafter, in which a pluralityof such conduits may extend within a single vessel.

To aid the gravity separation process within the vessel, the apparatusmay regiment the flow by comprising means for inducing a rotational flowin the fluid stream entering the vessel. This will allow efficientgravity separation and prevent cross-flow contamination of the separatedphases. This means is most preferably a tangential outlet in the conduitthrough which the fluid stream is introduced into the vessel. To furtheraid the separation, the vessel may further comprise means forcentralising the rotational flow of fluid within the vessel, for examplean inverted cone located coaxially within the vessel. The inverted conemay be provided with a fluid guide extending helical along its outersurface in the direction of fluid flow.

To remove the lighter fluid components from the heavier phases withinthe vessel, the apparatus may further comprise a conduit extendingcoaxially within the vessel, the conduit having openings therein throughwhich lighter fluid components may leave the fluid stream. In apreferred arrangement, the conduit has an outlet for the lighter fluidcomponents within the vessel, the outlet being disposed upstream of thefluid stream inlet.

The fluid remaining in the vessel will consist essentially of heavierfluid components. The apparatus may further comprise a heavier fluidcollection zone, a heavy fluid collection conduit being disposedcentrally within the collection zone, the conduit having a plurality ofopenings therein to collect the heavier fluid.

Should the fluid feed stream comprise any solid components, this willremain in the apparatus, passing in the downstream direction. In suchcases, the apparatus may further comprise a solids collection zone andmeans for removing solids from the collection zone, the means removingsolids on an intermittent or a continuous basis.

As mentioned above, the apparatus is particularly suited to beingconstructed on a modular basis. In particular, the assembly comprisingthe first and second conduits, and if present the conduit for housing afluid vortex and any means provided to increase the rotational velocityof the fluid stream, may be housed within a single conduit, representinga single separation assembly module. Accordingly, in a further aspect,the present invention provides an apparatus for separating a multiphasefluid stream comprising a heavier fluid component and a lighter fluidcomponent, the volume flowrate of the multiphase fluid stream beingsubject to variation over time, the apparatus comprising a plurality ofseparation assemblies as hereinbefore described and operable toaccommodate different fluid flowrates; the apparatus further comprisingmeans for selectively operating one or more separation assembliesaccording to the volume flowrate of the multiphase fluid stream.

The apparatus may be operated with one module, a selection of modules orall separation assemblies being in use. In this way, individualseparation assemblies may be brought on- and off-line as the volumetricflowrate of the stream varies. This is particularly effective when theindividual separation assemblies are sized to accommodate differentfluid flowrates. Preferably, the apparatus comprises a means for feedinga purge fluid to each separation assembly, in order to allow eachseparation assembly to be purged and cleaned before being brought on-and off-line.

If a gravity separation stage is required, the modular assembly mayfurther comprise a separation vessel as hereinbefore described, each ofthe separation assemblies extending within the separation vessel.

In one use of the modular apparatus of the present invention, aplurality of modular separation units may be provided, each comprising aplurality of separation assemblies of varying sizes. Such a group ofunits may be clustered around a wellhead location or in an oilfield, forexample at a surface or a subsea location, in order to serve a group ofwells.

The helical separation system of the present invention presents aparticular problem when it comes to start-up and shut-down, if theconduits for the lighter fluid produced in the process are not to becontaminated with heaver fluid components. This problem is solved by thestart-up and shut-down methods forming further aspects of the presentinvention.

Accordingly, the present invention provides a method for starting up ahelical separation system for operation in separating a multiphase fluidstream comprising a heavier fluid component and a lighter fluidcomponent, the method comprising feeding to the helical separationsystem a first fluid stream consisting essentially of the heaver fluidcomponent; when the fluid velocity within the helical separation systemhas reached the minimum operating velocity for the multiphase fluidstream, replacing over a period of time the first fluid stream with themultiphase fluid stream to be separated.

A method for shutting down a helical separation system from normaloperation in which a multiphase fluid stream comprising a heavier fluidcomponent and a lighter fluid component is being fed to the helicalseparation system, comprises the steps of introducing a first fluidstream consisting essentially of the heavier fluid component into themultiphase fluid stream feed to over time to replace the multiphasefluid stream; when the fluid feed consists of the first fluid stream,reducing the fluid feed flowrate to zero.

The helical separation system is preferably left full of the first fluidafter the fluid feed flowrate has been reduced to zero. In this way, theaforementioned start-up method may be employed without delay and in themost optimum manner to achieve normal operating conditions with theminimum of contamination of the lighter fluid streams.

The start-up and shut-down procedures of the present invention are ofparticular advantage when the helical separation system is arranged inthe modular format discussed hereinbefore and operated using a varyingselection of helical separation modules to accommodate different fluidflowrates and compositions.

Embodiments of the present invention will now be described, by way ofexample only, having reference to the accompanying drawings, in which:

FIG. 1 is a cross-sectional view of a complete separation apparatusaccording to an embodiment of the present invention;

FIG. 2 is a cross-sectional view of the upper portion of the separationapparatus of FIG. 1;

FIG. 3 is a plan view of the separation apparatus of FIG. 1;

FIG. 4 is a stylised cross-sectional view of a helical separationassembly according to the present invention;

FIGS. 5 a to 5 c are a stylised cross-sectional view on an enlargedscale of three portions of the helical separation assembly in theregions labelled as A, B and C in FIG. 4;

FIG. 6 is a longitudinal cross-sectional view of a helical separationassembly of the present invention;

FIG. 7 is a longitudinal cross-sectional view of the portion of theassembly of FIG. 1 along the line VII-VII;

FIG. 8 is a longitudinal cross-sectional view of the portion of theassembly of FIG. 1 along the line VIII-VIII;

FIG. 9 is a cross-sectional view of the upper portion of the separationapparatus of FIG. 1 along a different axis to that of FIG. 2;

FIG. 10 is a schematic representation of a system of the presentinvention, indicating how monitoring tubes are employed to monitor theperformance of various regions of the system;

FIG. 11 is a performance histogram showing the operating flow ranges andoperating pressure ranges for assemblies of differing dimensions; and

FIG. 12 is a graph indicating the selection of different assemblycombinations to accommodate different feed stream flowrates.

Referring to FIG. 1, there is shown a separation assembly, generallyindicated as 2. The assembly is shown arranged substantially verticallyat the seabed, with the lower portion of the assembly extending beneaththe seabed. This is one convenient arrangement for locating theassembly, in particular adjacent a subsea wellhead assembly. In thisway, existing wellhead assemblies can be provided with the separationassembly of the present invention, without significant modification ofthe wellhead installation.

The separation assembly 2, is formed around a generally cylindrical,tubular housing 4. The housing 4 is most conveniently a section ofcommercially available conductor casing. The conductor casing issupplied in a range of sizes, including the nominal sizes of 42 inches,36 inches, 30 inches and 20 inches (108 cm, 92 cm, 76 cm and 50 cm). Thehousing 4 may be constructed from a section of the conductor casing,with the diameter being selected to accommodate the volumetric flowrateof the fluid stream to be processed. The embodiments shown in theaccompanying figures and described hereinafter are concerned with fluidseparation at an undersea location. However, the method and apparatus,with only minor modifications, may also be applied to surface-boundconductors or to platform conductors.

The separation assembly 2 comprises a plurality of discrete components.An inlet and outlet assembly 6 is connected to the upper end of thehousing 4, for supplying fluid to the assembly for separation andthrough which the separated fluid streams are removed. The assembly 2further comprises a plurality of helical separation, assemblies 8extending within the housing 4, in which the first stage of separationof lighter fluid components from heavier fluid components is carriedout. The remaining portion of the housing 4 is arranged to providefurther stages of separation, comprising a fluid stabilisation region10, a second fluid-fluid separation stage 12, and a final fluid-solidseparation and recovery stage 14. Each of these components will bediscussed in more detail below.

An operating light fluid/heavier fluid interface 16, a maximum highlevel 17 and a minimum low level 18 for the fluid within the housing 4are represented in FIG. 1 and are shown as lying along the length of thehelical separation assemblies 8, such that they all He above the loweror downstream end of the helix assemblies.

Referring to FIG. 2, the inlet and outlet assembly 6 is mounted on theupper end of the housing 4 by means of a connector 20 of conventionaldesign. The inlet and outlet assembly 6 comprises a generallycylindrical inlet body 22. A cap assembly 24 is mounted on the upper endof the inlet body 22 and comprises a lower cap 26 and an upper cap 28,which together define a gas-liquid separation zone 30, in which gas isremoved from liquid present in the zone 30. A vent for the gas isprovided by means of a bore 32 extending obliquely through the upper cap28, which is in turn connected to a gas recovery conduit 34 by a flangeof conventional arrangement. A lateral bore 35 is formed in the lowercap 26 and connects to a fluid conduit 37. An axially central bore 36extends through the inlet body 22, connecting the inner region of thehousing 4 with a fluid mandrel 38 extending through the gas-liquidseparation zone, which in turn connects with a laterally extending bore40 through the upper cap 28. Liquid may be removed through thisarrangement, to be drawn into a liquid conduit 42, which is connected toa suitable line (not shown) by means of a conventional flange assembly.A coaxial bore 44 extends through the upper cap 28 and provides anopening for the removal of solid material, such as silt, from theassembly, for chemical injection, or for monitoring purposes. A valve 46is shown connected to the coaxial bore 44 in FIG. 2.

Turning again to the inlet body 22, a plurality of liquid conduits areprovided in the form of longitudinal bores 48 spaced around the centralbore 36. The liquid conduits provide a direct connection between thegas-liquid separation zone 30 and the upper region of the interior ofthe housing 4, through which fluids may pass, as required.

The inlet body 22 is provided with a further set of longitudinal bores50 spaced around and radially outwards of the central bore and thelongitudinal bores 48. As will become apparent, the bores 50 provide thefeed conduit for each helical separation assembly 8. Each of thelongitudinal bores 50 is connected at its lower opening to a respectivehelical separation assembly 8, details of which are providedhereinafter. The arrangement of the longitudinal bores 50 are theirassociated helical separation assemblies 8 is shown in plan view in FIG.3. As shown in FIG. 3, each of the longitudinal bores 50 is providedwith a tangentially arranged inlet 52, from which extends a radial bore54. An inlet conduit 56 is connected to the end of each radial bore 54by means of a conventional flange assembly. Each inlet conduit 56 isconnected to a fluid inlet header 58, shown in FIG. 3 as a circular pipeextending around the upper portion of the assembly. A fluid feed conduit60 connects to the fluid inlet header 58, through which a multiphasefluid stream to be processed may be fed. The flow of fluid from thefluid inlet header 58 to each inlet conduit 56 is controlled by way of avalve 62. As shown in FIG. 3, each fluid inlet conduit 56 is providedwith its own valve 62 and a one-way check valve 63 to prevent any backflow occurring. This provides for independent control of each fluidinlet conduit 56 and the flow of fluid to each helical separationassembly 8. In this way, the assembly is operable to accommodate thegreatest variations in the flowrate and composition of the fluid feedstream. It will be appreciated that alternative arrangements arepossible, in which a single valve is used to control the flow of fluidto two or more helical separation assemblies 8, albeit with a reductionin the freedom of operation. Such an arrangement may be employed, forexample, in situations where only limited variations in the flowrateand/or composition of the fluid feed stream are anticipated during theworking life of the installation.

Radial ports 55 extend through the inlet body 22 and connect withrespective lines 53, which extend to an appropriate position within thehousing 4. These sire employed for fluid injection, fluid sampling orprocess monitoring operations.

A fluid purge system is also shown in FIG. 3 and comprises a similararrangement to the fluid inlet system described above, including acircular purge fluid header 64 having a purge inlet conduit 66 extendingto each fluid inlet conduit 56. The operation of the fluid purge systemis to provide a flow of purge fluid, typically water, to each helicalseparation assembly, as it comes on- and off-line. A valve 68 ispositioned in each purge inlet conduit 66, in order to provideindependent control of the purging of each helical separation assembly8. Again, two or more helical separation assemblies 8 may have theirpurging controlled by a single valve. A purge fluid feed conduit 70supplies purge fluid to the purge fluid header 64.

While referring to FIG. 3, it is convenient to note the arrangement ofthe helical separation assemblies 8. As shown, the assembly comprises atotal of 10 helical separation assemblies 8 of a range of sizes, able toaccommodate a range of different fluid flowrates. In the arrangementshown, the assembly comprises one each of a helical separation assembly8 having a nominal diameter of 4 inches, 5 inches and 6 inches (10 cm,12.5 cm, and 15.25 cm). In addition, the assembly comprises 7 helicalseparation assemblies 8 having a nominal diameter of 7 inches (18 cm).The arrangement shown can thus be operated over a wide range of feedfluid flowrates, from the lowest flowrate when the single 4 inch helicalseparation assembly is on-line, up to a maximum flowrate when allhelical separation assemblies are operating. Combinations of the helicalseparation assemblies 8 may be made to accommodate flowrates betweenthese two extremes.

The arrangement shown in FIGS. 1 to 3 is one in which the feed and purgeheaders and their respective valves are integral with the inlet andoutlet assembly 6. It will be appreciated that an alternativearrangement may be employed, in which the valves and headers arecombined in a separate module that is connected to the inlet and outletassembly 6 by suitable lines. In this way, the valves and their controlpipework may be more readily accessible for retrieval and replacement.

It is a significant advantage of the assembly that the number and sizeof the helical separation assemblies arranged within the housing may bevaried to accommodate a particular duty, allowing the design andconstruction of the overall assembly to be on a largely modular basis.This in turn allows the design, construction, maintenance and repair tobe both straightforward and economical.

The construction and operation of the helical separation assemblies 8will now be described, having reference to FIG. 4, which is a stylisedrepresentation of a typical assembly. It will be appreciated that theassembly shown in FIG. 4 is significantly shortened, for ease ofreference, the ratio of the overall length to diameter of the helixassembly typically being much greater than that represented in FIG. 4.

Referring to FIG. 4, a helical separation assembly 8 comprises agenerally cylindrical conduit 100, shown in FIG. 4 to extend verticallydownwards from the inlet body 22. A light fluid conduit 102 in the formof a generally cylindrical tube extends coaxially within the cylindricalconduit and is open at its uppermost end into the gas-liquid separationzone 30 in the cap assembly 24. An annular cavity is formed around thelight fluid conduit 102 between the light fluid conduit 102 and thecylindrical conduit 100. The uppermost region 104 of the annular cavityis empty, allowing for the free passage of fluid. One of the radialbores 54 in the inlet body 22 terminates in a tangential opening 52 inthe uppermost region 104 of the cylindrical conduit 100.

The region of the annular cavity adjacent and below the uppermost region104 is a fluid flow stabilisation region, indicated as 106 in FIG. 4. Inthis region, a helix 108 is disposed within the annular cavity andextends around the light fluid conduit 102, to form a helical flowpathfor fluid moving within the cylindrical conduit. The function of thisregion is to allow the flow of fluid to stabilise into the required flowregime, by forcing the fluid to flow in a compact helical path. Thehelix in the flow stabilisation region 106 is formed to provide a stablefluid flow pattern and to allow the phases to centrifugally divide andpart, being subjected to multiple gravity and rotational forces beforethe outlet. In the arrangement shown in FIG. 4, the cross-sectional areaof the helical flow path is preferably constant along the entire lengthof the helix 108. As the helix 108 is disposed within a cylindricalconduit 100, this determines that the pitch of the helix 108 ispreferably constant along the length of the region 106. In otherarrangements, the pitch of the helix 108 may be varied along its length,in order to provide the required flow pattern at its outlet end.

The movement of droplets of the light fluid phase in the helical fluidflow stabilisation section 106 is represented in FIGS. 5 a to 5 c.

The end of the flow stabilisation region 106 and the helix 108 iscontiguous with a fluid separation region, generally indicated as 110.In this region, a helix 112 is disposed within the annular cavity andextends around the light fluid conduit 102, to form a helical flowpathfor fluid moving within the cylindrical conduit. The function of thisregion is to separate the lighter fluid phase from the heavier fluidphase. The light fluid conduit 102 is provided with a plurality of portsor holes 114. The ports 114 are formed in the inner upper region of thehelical flowpath. The light liquid phase is recovered through the ports114 in the fluid conduit 102 as described hereinafter.

In order to provide the separation of fluid phases in the fluidseparation region 110, the cross-sectional area of the helical flowpathis increased along the length of the region 110. In order to providethis increase, the helix 112 is shown in FIG. 4 as increasing in pitchalong the length of the fluid separation region 110. The increase isshown as being about a 1% increase in the pitch of the helix 112 foreach complete turn around the light fluid conduit 102. The increase inpitch is to allow natural flow of the fluid (as opposed to the forcedflow in the upstream helical sections) and to prevent a fluid backpressure arising due to friction forces within the helical channel. Ifallowed to occur, the fluid back pressure would give rise to adetrimental cross-flow force within the fluid. The increase in the pitchwill depend upon the properties of the fluid being processed and isselected to allow the natural movement of the lighter phases into thefluid conduit to occur, while allowing the remaining heavier fluidphases to continue along the helical flowpath.

The end of the fluid separation region 110 and the helix 112 iscontiguous with a fluid velocity enhancing region, generally indicatedas 116. In this region, a further helix 118 is disposed within theannular cavity and extends around the light fluid conduit 102, to form ahelical flowpath for fluid moving within the cylindrical conduit. Thehelix 118 terminates at the open end of the light fluid conduit 102. Thefunction of this region is to increase the velocity of the fluidremaining in the cylindrical conduit 100 so as to provide a stablevortex in the downstream or lower region of the conduit 100 as describedbelow.

In the fluid velocity enhancing region 116, the helix 118 is shown inFIG. 4 increases in pitch for each turn around the light fluid conduit102. The increase in the pitch will be determined by the nature of thefluids be separated and the specific separation duty to be performed. Atypical rate of increase of the pitch of the helix is about 3% for eachturn around the light fluid conduit 102. The portion of the light fluidconduit 102 extending through the region 116 is typically cylindricaland of a substantially constant diameter. An alternative embodiment isto provide the light fluid conduit 102 with a tapered or flared portion,such that its diameter increases through this region in the direction offluid flow in the annular cavity. This in turn causes the annular cavitybetween the light fluid conduit 102 and the cylindrical conduit 100 toreduce in cross-sectional area in the downstream direction of fluid flowin the annular cavity.

A cross-sectional view of a typical entire helical separation assembly 8is shown in FIG. 6, from which it will be appreciated that manyseparation operations require the length of the three regions 106, 110and 116 to be many times greater than the diameter of the cylindricalconduit 102. It will also be noted that the helices 108, 112 and 118 areshown as a single helical element extending within the cylindricalconduit 102. This is a preferred arrangement. However, it will beappreciated that each of the helices 108, 112 and 118 may be arrangedseparately within its own portion of the cylindrical conduit 102, oreven within separate conduits. The arrangement shown in FIGS. 4 and 6 isadvantageous when applying the helical separation assembly 8 on amodular basis, as described. For certain fluid separations, double helixarrangements may be employed, comprising two helical paths between theconduits 100 and 102.

As noted above, the helix 118 within the fluid velocity enhancing region116 terminates at the end of the light fluid conduit 102. Thecylindrical conduit 100 is provided with an oriented and angled outlet120 at its lower, downstream end, details of which are describedhereinafter. The downstream portion of the cylindrical conduit 100extending from the end of the light fluid conduit 102 to the outlet 120of the cylindrical conduit 100 is a substantially empty volume andprovides a vortex region, generally indicated as 122. As will bedescribed below, a vortex is established in this region extending in thedownstream direction from the end of the light fluid conduit 102. Tocapture the created vortex, a vortex guide, in the form of an invertedcone 124 and a vortex tube 126 are disposed within the end portion ofthe light fluid conduit 102, as shown more clearly in FIG. 6.

The helical separation assembly shown in FIGS. 4 and 6 may be operatedas a self contained separation system. Alternatively, if furtherseparation, such as polishing, is required, the helical separationassembly may be used in conjunction with the further separation systemsand method described below, most suitably in an assembly as shown in theaccompanying figures.

Referring to FIG. 7, there is shown a cross-sectional view of theportion of the assembly of the embodiment of the present inventionimmediately downstream of the outlet 120 of the helical separationassembly 8. As shown in FIG. 7, a plurality of helical separationassemblies 8 (two of which are visible in FIG. 7) extend downwardswithin the housing 4 and are retained in position by a baffle plateguide assembly 123. A plurality of baffle plate guide assemblies 123 maybe provided, depending upon the length of the helical separationassemblies 8 and their relative dimensions. The baffle plate assemblies123 provide guidance and spacing to the helical separation assemblies.In addition, they serve to disperse any large gas bubbles that may bepresent in the bulk fluid, as a result of gas floatation being employed.

The outlet 120 of each helical separation assembly 8 is oriented so asto direct fluid leaving the conduit in a downwards tangential direction,in order to create a wide vortex flow regime, as described below. Asecondary light fluid conduit 160, in the form of a generallycylindrical tube, extends coaxially within the housing 4 and has itsupper end open to form an outlet 162, It will be noted that the outlet162 is above, the outlet 120 in the lower end of each helical separationassembly 8. The rotating flow below the outlets will initially consistof cross-flows, until it has stabilised. The light fluid conduit 160acts as a cowling to assist any light fluid droplets to move up throughthis unstabilised fluid region 10.

A heavy fluid conduit 164 extends coaxially within the secondary lightfluid conduit 160 up to and coaxially through the fluid mandrel 38,which is connected at its upper end to the liquid conduit 42, as shownin FIG. 2.

A solid/injection conduit 166 extends coaxially within the heavy fluidconduit 164 and is connected at its upper end to the coaxial bore 44 inthe upper cap 28, as shown in FIG. 2. The flow of material through thesolid/injection conduit 166 is controlled by the valve 46, also shown inFIG. 2.

An inverted cone 170 is disposed around the secondary light fluidconduit 160 and spaced from the lower ends of the helical separationassemblies 8. A helical vane 172 is provided on the conical surface ofthe inverted cone 170. The region within the housing 4 between the lowerends of the helical separation assemblies 8 and the downstream or lowerend of the inverted cone 170 is a fluid flow re-stabilisation region,generally indicated as 10 in the figures, the purpose of which is toestablish a slower rotational flow pattern of fluid flowing downwards inthis region from the outlets 120 of the helical separation assemblies 8.The inverted cone 170 is of such a length and angle to provide asufficient reduction in the annular flowpath between the housing 4 andthe secondary light fluid conduit 160 to create a higher rotationalannular velocity of the fluid to effect a final separation of thelighter and heavier fluid phases in the next region of the assembly.

The region of the housing 4 immediately downstream or below the invertedcone 170 is a second fluid-fluid separation stage, generally indicatedas 12 in the figures. In this region, the remaining lighter fluid phasesare finally removed from the assembly. To achieve this, the secondarylight fluid conduit 160 is provided with a plurality of ports or holes174 along its length from inside the inverted cone 170, through whichthe lighter fluid phases may enter the conduit 160 and pass along theannular cavity between the light fluid conduit 160 and the heavy fluidconduit 164. It will be noted that the secondary light fluid conduit 160is closed at its lower end 161, as shown in FIG. 8.

Referring to FIG. 8, there is shown a cross-sectional view of thedownstream or lower portion of the separation assembly. The final regionin the assembly is the final fluid-solid separation and recovery stage,generally indicated as 14 in the figures. A conical vane 176 is disposedat the downstream end of the second fluid-fluid separation stage 12 andprovides a barrier to the lighter fluids in the central annulus andensures only the heavier fluid components and solids in the outerannulus continue in the downwards flow direction. The conical vane 176marks the upstream end of the final fluid-solid separation and recoverystage 14. The portion of the heavy fluid conduit 164 extending into thisfinal region 14 is perforated by a plurality of ports 180, through whichthe heaviest fluid phases are withdrawn.

The solid/injection conduit 166 extends to the end region of the heavyfluid conduit 164, as shown in FIG. 8. Means may be provided to withdrawsolid material, such as silt and sediment, through the solid/injectionconduit 166, for example by means of a reduced pressure or vacuumsuction. Alternatively, the solid/injection conduit 166 may be used toinject active components into the fluid in the housing, for example toenhance the separation of the fluid phases.

The lower end of the housing 4 is provided with a bore 182, in which arelocated an isolation plug or plugs 184 and a check valve 186, both ofconventional design. The bore 182 may be used to provide jetting orcirculation for seawater, muds or cements when installing the housing 4into the seabed.

The operation of the assembly in the accompanying figures will bedescribed in relation to the separation of a two phase mixture of oiland water. Such a mixed phase stream is typical of the water recoveredfrom the production fluids of a subterranean well. Typically in such astream, the oil is suspended as droplets in the bulk aqueous phase,which are not susceptible to coalescence and separation using theconventional techniques of gravity separation and are of insufficientmass to segregate under low centrifugal forces. To be suitable forreinjection into an underground formation, the oil must be removed fromthe water to a concentration below 400 ppm. This is achieved using themethod and apparatus of the present invention in the embodiment shown inthe accompanying figures as follows:

The mixed phase oil/water stream is fed to the assembly 2 through thefluid feed conduit 60 and enters the fluid feed header 58, from where itis distributed to one or more helical separation assemblies 8 throughthe respective inlet conduit 56, the flow through which is controlled bythe respective valve 62 and one-way check valve 63. This allows thegeneral flow to be segregated and divided into manageable portions fordistribution to respective helical separation assemblies. As describedabove, the number and combination of the helical separation assemblies 8to be used is selected to match the volumetric flowrate of the feedstream to be processed. As noted, it is an advantage of the assembly ofthe present invention, in particular as shown in the accompanyingfigures, that a wide range of volumetric flowrates may be accommodatedwithout any reduction in the efficiency of separation. Indeed, theability to select a combination of different sized helical separationassemblies allows the system to be tailored to a very wide range offluid compositions and flowrates, while allowing the separationprocesses to operate under their optimum conditions and at a highefficiency.

From each inlet conduit 56, the oil/water stream enters the respectiveradial bore 54 in the inlet body 22, through the tangential opening 52in the uppermost portion of the cylindrical feed conduit of therespective helical separation assembly 8, as shown in FIGS. 3 and 4. Theselected fluid stream enters the cylindrical feed conduit tangentially,where coarse cyclonic separation occurs. This allows the general phasemasses to be divided and begin to separate and to perform the streaminto a plurality of discrete phases before the fluid enters the helicalflowpath. As the operation of each helical separation assembly isidentical, with the only difference being the size of the assembly andits volumetric throughput, the operation of just a single helicalseparation assembly 8 will be described for clarity.

The oil/water stream entering the helical separation assembly 8 iscaused to flow in a rotating pattern as it descends the uppermost region104, as viewed in FIG. 4. The oil/water then enters the fluid flowstabilisation region 106 and enters the helical flowpath formed by thehelix 108. The function of the uppermost region 104 and the fluid flowstabilisation region 106 is to generate a uniform, rotating fluid flowpattern in the oil/water stream. The passage of the stream through thevarious conduits and pipes upstream of the separation assembly 2 willprovide the stream with a turbulent flow regime, in which the Reynoldsnumber is significantly above the critical Reynolds number upon entryinto the helical separation assembly 8. Such a turbulent flow patternwill not provide a high efficiency of separation of oil droplets fromthe water. Accordingly, the uppermost region 104 and the fluidstabilisation region 106 are operated to stabilise the flow regime suchthat the Reynolds number is below the critical number. In other words,the Reynolds number of the fluid flow is brought below the value atwhich turbulent flow arises. Preferably, the flow stabilisation region106 is of sufficient length for the fluid flow regime to become laminar.At least, the flow regime should be in the transitional state,preferably with a Reynolds number towards the lower end of thetransitional range.

In the flow stabilisation region 106, the transitional state and thecompact helical flow pattern will generate high centrifugal forceswithin the fluid, forcing even the smallest droplets of fluid to migrateaccording to their respective densities. This action encouragescoalescing of the small droplets into larger drops, which in turn, dueto their larger masses, experience a larger force and accelerate theseparation of the phases. An advantage of a forced flow in the flowstabilisation region 106 is that it significantly increases the criticalReynolds number, allowing the Reynolds number to be considerably higherbut still within the laminar flow regime than for flow in an openstream. This in turn allows the fluid to flow at a significantly highervelocity along the helical path.

Upon leaving the fluid flow stabilisation region 106 the oil/waterstream immediately enters the upper end of the fluid separation region110 and the helical flowpath formed by the helix 112. In this region,the major portion of the oil droplets are large enough to collect and tobe separated from the water in the oil/water stream and removed from thestream. The action of the high centrifugal forces on the minute oildroplets and separation action at various stages as the flow is forcedthrough the helix is represented diagrammatically in FIGS. 5 a to 5 c.In the upper regions of the fluid separation region 110, as shown inFIG. 5 a, the rotational flow of the fluid stream causes the lighter oildroplets to migrate to the upper, inner region of the helical flowpath,as viewed in FIG. 5 a. This movement progresses along the length of thehelical flowpath, as shown in FIGS. 5 b and 5 c. The oil collecting inthe upper, inner region of the flowpath flows through the ports 114 inthe light fluid conduit 102 and passes upwards in the conduit to thegas-liquid separation zone 30 in the cap assembly 24, as shown in FIG.2. Any gas present in the oil at this point is collected in the upperregion of the gas-liquid separation zone 30 and is removed through thebore 32 and the gas recovery conduit 34. The oil is removed from the capassembly 24 through the lateral bore 35 in the lower cap 26 and thefluid conduit 37. Any water entrained with the oil and reaching the capassembly 24 returns to the housing 4 by way of the longitudinal bores 48in the inlet body 22.

Upon leaving the fluid separation region 110 the remaining liquid,consisting essentially of water with minor amounts of oil, enters thefluid velocity enhancing region 116 and the upper end of the helicalflowpath provided by the helix 118. In this region, the rotationalvelocity of the stream is increased. As a result, the Reynolds number ofthe stream increases and may approach the critical value. The velocityof the stream is increased sufficiently to produce a stable vortex inthe portion of the cylindrical conduit 100 immediately downstream of theend of the fluid velocity enhancing region 116. The vortex is stabilisedat the open end of the light fluid conduit 102 and collected with theaid of the inverted cone 124 and the vortex tube 126 in the lower end ofthe light fluid conduit 102. Under the action of the rotational movementof the fluid in the vortex, the remaining oil droplets migrate to thecentre of the vortex and enter the lower end of the light fluid conduit102, from where it passes to cap assembly 24, as discussed above.

The remaining liquid flows down the cylindrical conduit 100 and leavesthrough the angled outlet 120 to enter the main volume of the housing 4.In operation, the main body of the housing 4 is filled with liquid, thelower region being filled with water and the upper region being filledwith the lighter oil. The entire assembly is operated such that theoil/water interface is above the maximum high level 17 of thecylindrical conduit 100 of the helical separation assembly 8. In themain volume of the housing 4, two actions enhance the separation of anyremaining oil droplets from the water. The first action is astraightforward gravity separation, by which the lighter oil dropletsare caused to rise within the housing and enter the upper region. Theoil collected in this region will leaving the housing 4 through thelongitudinal bores 48 in the inlet body 22 to enter the gas-liquidseparation zone 30 in the cap assembly 24. The oil is removed from thecap assembly 24 as described above.

The second mode of separation in the main volume of the housing is afurther rotational separation. The action of the angled outlet 120 is toinduce a slow rotation of the substantially water phase within the lowerregion of the housing 4. The rotating water stream descends within thehousing through the fluid stabilisation region 10. As the water streampasses the inverted cone 170 and the helical vane 172, its rotationalvelocity is increased, before the water stream enters the furtherfluid-fluid separation region 12. In this region, the remaining oildroplets are caused to migrate to the centre of the housing 4, wherethey pass through the ports 174 in the secondary light fluid conduit160. Within this conduit, the oil droplets move upwards past the outlets120 of the helical separation assemblies 8 and enter the upper region ofthe housing 4.

The water leaving the further fluid-fluid separation region 12 willcontain only very minor or trace amounts of oil and be suitable forreinjection into a subterranean formation or for disposal in other ways.The water is removed from the assembly in the removal region 14 bypassing through the ports 180 in the heavy fluid conduit 164. The waterin this conduit flows upwards to the cap assembly 24 and leaves theassembly 2 through the lateral bore 40 and the liquid conduit 42.

Any solid materials, such as sediment or silt, may be collected in thelowermost region of the housing 4 and removed, either periodically orcontinuously, through the solid/injection conduit 166.

The solid/injection conduit 166 also provides a means for introducingcomponents into the fluids in the housing 4, such as separationenhancers, as may be required to improve the separation efficiency ofthe overall assembly.

A portion of the water removed from the fluid removal section 14 may berecycled to the inlet conduit 60, in order to adjust the volumetricflowrate of fluid through the assembly. This may be needed, for example,to provide sufficient rotational velocity of the oil/water streams inthe helical separation assemblies 8.

The control and monitoring of the overall system is achieved using anarrangement of injection, monitoring and sample lines. As noted above,the cylindrical inlet body 22 is formed with a plurality of radial bores55 connected at their inner ends to respective control lines 53. As moreclearly shown in FIG. 9, the control lines 53 extend longitudinally in adownstream direction within the housing 4. The radial bores 55 and thecontrol lines 53 may be used to inject components into the bulk fluidphase within the housing, such as additives and separation enhancers.Gas may be injected through one or more of these lines in order toprovide a gas floatation system within a liquid bulk phase.

One particular use for the control lines 53 is to determine and monitorthe interface between the light fluid phase and the heavy fluid phasewithin the housing 4. As described above, the light fluid phase will becollected from and rise upstream within the housing to occupy theuppermost regions of the housing, as shown in the Figures. For efficientoperation of the separation process, it is necessary to identify theinterface between the two phases. In operation, this may be a welldefined interface 15. Alternatively, depending upon the nature of thefluids concerned, the interface may be poorly defined. For example, inthe case of the separation of oil dispersed in a continuous aqueousphase, the interface may extend over several meters and comprise anemulsion of oil and water.

The technique of determining the position of the interface 15 is shownschematically in FIG. 10. A control line 53 is shown extending into thehousing 4, the lower end of which defines a datum 19. The pressure Ps ofinjected fluid in the control line 53 is measured by a sensor.Similarly, the pressure Ph within the housing at its uppermost end ismeasured. To determine the interface 15 in an oil/water fluid system,clean oil, for example that removed from the light fluid conduit afterseparation in the assembly, of a known density is introduced into one ormore of the control lines 53. The pressure in the control line ismeasured and compared with the pressure at the exit of the light fluidconduit. The height h_(w) between the datum 19 and the interface 15 isdetermined using the formula:

$h_{w} = \frac{\left( {P_{s} - P_{h}} \right)}{\left( {d_{w} - d_{o}} \right)}$

where h_(w) is the height between the datum and the interface 15; P_(s)is the pressure of injected oil in the control line 53; P_(h) is thepressure in the uppermost end of the housing 4; d_(w) is the density ofwater; and d_(o) is the density of the injected oil. A similar formulais applied to other fluid systems.

A constant feed of light fluid, such as oil, is maintained through thecontrol line 53, in order to allow the system to actively monitor thechanges in the interface. In general, the system will be operated with apredetermined operating level 16, as shown in FIG. 10, with a high point17 and low point 18 for the interface, defining the acceptable operatingrange of the fluid/fluid interface. Movement of the system outside ofthis range can be used to trigger an alarm and/or initiate a correctiveoperation, such as the injection through one or more control lines 53 ofa volume of light fluid or heavy fluid. Alternatively, or in addition,one or more of the outlet pumps or inlet chokes around the system may beadjusted, depending upon the correction required. The corrective actionmay include recycling a portion of the heavier fluid.

As noted above, the arrangement of the present invention is particularlysuited for application on a modular basis. In one preferred arrangement,a separation module comprises a helical separation assembly, asdescribed both in general and in specific detail above and shown in theaccompanying figures, indicated by the general reference numeral 8. Thehelical separation assembly may be provided in a variety of differentsizes, in particular a range of different nominal diameters. Thispossible variation in the size of the separation module is an advantageof the present invention by allowing a wide range of fluid flowrates andcompositions to be accommodated. There are a number of ways in which themodular approach of the present invention may be applied.

First, the larger size helical separation assemblies can accommodatelarger fluid flowrates. Referring to FIG. 11, there is shown a graph ofthe operating flowrates and pressures for a range of helical separationassemblies 8 as shown in the accompanying figures. The operating rangesand parameters are given for helical separation assemblies havingnominal diameters of 4, 5, 6 and 7 inches (numbered 1 to 4 in FIG. 11)and for operation in the separation of crude oil droplets from a waterstream. Such a stream is typical of the oil-contaminated water streamsencountered during drilling and production operations in subterraneanoil and gas wells. As a first approach to accommodating a given fluidstream and flowrate, it is merely necessary to select the appropriatesize of helical separation assembly, for example from a graph such asFIG. 11.

If the stream to be processed has a flowrate exceeding the maximumoperating flowrate of the helical separation assembly, the stream may besplit and a plurality of such assemblies may be operated in parallel. Afurther manner to apply the modular approach of the present invention isto select a plurality Appropriate selection of the sizes of theplurality of helical separation assemblies allows a combination ofdifferent sized assemblies to be determined to match the given streamand flowrate.

A complication arises when the flowrate and/or composition of the fluidstream to be processed will vary as the well or wells are brought onstream or shut down and over the working lifetime of the separationassembly. This situation is likely to be encountered in the case ofoffshore oil and gas wells. It is preferred to provide equipment at suchremote locations that can operate for extended periods of time,typically many years, with little or no adjustment or maintenance Aproblem arises with separation equipment at such remote locations as aresult of the fluid flowrate and composition produced from the wellvarying over time. Advantageously, the present invention provides aseparation system that can be installed and operated to accommodate arange of flowrates and compositions changing over time.

An assembly incorporating the concepts of the present invention andadapted to accommodate such changes in the fluid stream over timecomprises a plurality of helical separation assemblies 8 of a variety ofnominal sizes. As the fluid flowrate and compositions change, theindividual helical separation assemblies are brought on- and off-line inthe appropriate combination to be matched to the fluid stream beingprocessed and provide optimum separation efficiency. Referring to FIG.12, there is shown, as an example, a further graph in which the flowrateof an oil-contaminated water stream, such as obtained from theproduction of oil from a subterranean well, is matched with combinationsof the helical separation assemblies 1 to 4 of FIG. 11. As will be seen,at low flowrates, a single helical separation assembly of theappropriate size can be employed. As the flowrate increases, it isnecessary to employ combinations of two or more assemblies of theappropriate size. The number and combination of sizes of separationassemblies are selected to match the required total flowrate, whilestill allowing each individual assembly to operate within its operatingrange and at its optimum efficiency.

Referring to FIG. 11, there is shown a dual vertical axis histogram. Thevertical axis 200 on the right hand side indicates the fluid flowratefor each helix, while the vertical axis 202 on the left hand side showsthe pressure differential across a helix. The base of the histogramidentifies a single size helical separation assembly. For each helicalseparation assembly, the vertical column 204 on the left depicts theminimum flowrate to achieve sufficient centrifugal forces within thefluid and the maximum flowrate 206 acceptable to remain below thecritical Reynolds number.

The column 208 on the right of each helical separation assembly showsthe minimum differential pressure allowable to achieve acceptablecentrifugal separation within the flow and the maximum differentialpressure 210 to remain below the critical Reynolds number. Failure tooperate with the flowrate at the correct pressure differential withinthe operating band for each helical separation assembly will result inlight fluid being carried through the system and polluting the heavierfluid phase collected. This will render the heavier fluid unacceptablefor pumping downhole, unless it is recycled to the inlet of theseparation system and the lighter fluid phases removed.

Therefore, as the total flowrate increases or decreases, helicalseparation assemblies cannot be simply opened or closed, as the fluidflow to other open helical separation assemblies could change and beoutside the aforementioned operating windows. For the overall system toperform the required separation duty over a wide range of fluidflowrates, intermediate helical separation assembly combinations have tobe selected.

Turning specifically to the examples of FIG. 11, at very low flowrates,that is below 1200 BPD, a single helical separation assembly, number 1in FIG. 11 having a nominal diameter of 4 inches is applied. The optimumoperation of the single assembly is achieved using a recirculation ofclean water to supplement the low flowrate of the stream to beprocessed. As the flowrate of the stream to be processed increases,helical separation assemblies 2, 3 and 4 are brought on-line, eitheralone or in combination. Flowrates of up to 5500 BPD may be accommodatedusing a single helical separation assembly 4, having a nominal diameterof 7 inches. To be capable of covering a full flow range will requirecombinations of two or more of the assemblies illustrated in FIG. 11 tobe employed.

FIG. 12 shows a histogram with the vertical axis 212 indicating thetotal fluid flowrate to be accommodated by the assembly 2. The base ofthe histogram identifies the maximum flowrate step 220 and the minimumflowrate step 222 that can be processed using the combination of helicalseparation assemblies 224 shown in the steps identified as 225. Eachcombination of helical separation assemblies has been numbered and theindividual assemblies making up the given combination identified. Thesafe operating range 226 for each assembly combination has beenindicated. Thus, a flowrate of 42,400 BPD is accommodated using acombination of seven helical separation assemblies of nominal diameter 7inches (assembly 4 in FIG. 11) and one assembly of nominal diameter 6inches (assembly 3 in FIG. 11), identified as assembly combination 16 inFIG. 12.

As will be seen in FIG. 12, the safe operating range 226 selected foreach assembly combination overlaps the operating range of the twoadjacent combinations. As the total fluid flowrate increases and themaximum operating flowrate of a given assembly combination 228, theoperation is switched to the next higher assembly combination, asidentified in the steps 225. When the total fluid flowrate drops andapproaches the minimum operating flowrate 230 of the combination,operation is, switched to the next lower assembly combination. In thisway, a smooth, continuous fluid separation operation can be achievedfrom zero fluid flowrate to the maximum total throughput 232 of thecomplete assembly 2.

As will be appreciated, during the operation of the assembly of thepresent invention when applied in a modular approach, individual helicalseparation assemblies are brought on- and off-line, as the fluidflowrate and composition changes. This requires each assembly to bestarted and shut down. Preferred methods for starting the assemblies andshutting them down are provided as aspects of the present invention.

In order for the required separation to be achieved in the variousseparation stages of the present invention, it is necessary that thefluid flowrate is above a critical minimum value, as indicated by thevalue 204 for each helical separation assembly shown in FIG. 11. At flowrates below this critical value, the lighter fluid phases will be not becompletely removed and will contaminate the heavier fluid phasesproduced in the process. This characteristic makes it undesirable simplyto shut down and start up the individual separation assemblies usingjust the fluid stream to be separated. To overcome this problem, it ispreferred to bring each helical separation assembly on line using apurge of clean heavy fluid.

As noted above and as shown in FIGS. 2 and 3, the inlet assembly of thesystem comprises a purge fluid header 64 fed by a purge fluid conduit70. The flow of purge fluid from the header 64 to each helicalseparation assembly 8 is controlled by a purge fluid valve 68. When thepurge fluid valve 68 for a given helical assembly 8 is opened, clearpurge fluid (such as clean water in the case of an assembly separatingoil droplets from produced water) is introduced downstream of the valve62 and check valve 63 controlling the flow of fluid to be processed. Atstart-up of a given helical separation assembly, the relevant valve 62and check valve 63 are closed and the purge fluid valve 68 opened, toprovide a stream of fluid above the critical minimum flowrate forseparation. Once the flow has been established, the valve 62 is opened.It is preferable to have the purge fluid pressure above the fluid streampressure, in order to ensure that the purge fluid stream has dominanceover the fluid stream being processed. In this way, when the valve 62 isopened, the fluid stream will not flow, as the higher purge fluidpressure will keep the check valve 63 closed. As the purge valve 68 isgradually closed, the pressure of the purge fluid entering the radialbore 54 will fall, allowing the check valve 64 to open. In this way, thefluid stream to be processed replaces the diminishing purge fluid, asthe purge valve 68 closes, bringing the helical separation assembly 8fully on line.

To shut down a given helical separation assembly, the opposite procedureis followed. Thus, the appropriate purge fluid valve 68 is graduallyopened, supplementing the fluid stream with purge fluid. The check valve63 prevents a higher pressure down stream of the valve 62 entering thefeed fluid system and flowing upstream. The purge fluid, being at ahigher pressure, will gradually close the check valve 63, in turn slowlyshutting off the flow of feed fluid, until static flow is achieved. Thevalve 62 is then closed at this point.

The purge valve 68 remains open until sufficient fluid has passed tocompletely purge the helical separation assembly 8 of all residual fluidbeing processed. The purge valve 68 is then closed. The helicalseparation assembly 8 is left containing only clean purge fluid and maybe left off line in this state until such time as a further change inthe total fluid flowrate requires it to be brought on line.

It is important that the helical separation assemblies 8 are broughtoffline in a cleaned and purged state, as the start up flowrate of fluidthrough the assembly 8 will be below the critical minimum flowrate toachieve complete separation. If the helical separation assembly is leftcontaining fluid being processed, this would be flushed into thedownstream clean fluid zone upon start up. This would result incontamination of the separated fluid fractions. This contaminated fluidwould need to be recycled to the inlet of the assembly 2 to be processedagain. This could result in the wellhead production flow having to bereduced or even shut off, until the contaminated fluid has beenprocessed. As will be appreciated, this is not acceptable for thecontinuous well production process, in particular given the frequencythat the combination of helical separation assemblies 8 being brought onand off line would need to change.

The purge fluid feed valve 68 may be a choke, flow control valve, ballor gate valve. The helical separation assembly may be maintained in thisstate until needed to be brought on-line again.

The operation of the present invention, in particular the embodimentsshown in the accompanying figures, has been described in detail inrelation to a multiphase stream comprising oil and water. It will beunderstood that the assembly and method of the present invention may beemployed to separate other multiphase liquid-liquid streams. The streammay contain two, three or more phases, which may be separated accordingthe relative densities of the liquids concerned. In addition, theinvention may be employed to separate multiphase gas-liquid streams in asimilar manner.

1. A method for separating a multiphase fluid stream including a heavier fluid component and a lighter fluid component, the method including: causing the fluid to flow along a helical flowpath in which the critical Reynolds number of the fluid flow is elevated, the fluid stream flowing at a Reynolds number below the elevated critical number, the fluid stream flowing at a sufficient velocity to cause the fluid phases to separate.
 2. The method according to claim 1, wherein the critical Reynolds number is greater than 10,000.
 3. The method according to claim 2, wherein the critical Reynolds number is greater than 100,000.
 4. A method for separating a multiphase fluid stream including a heavier fluid component and a lighter fluid component, the method including: causing the fluid to flow along a helical flowpath extending around a central conduit, the fluid flowing at a sufficient velocity to cause the lighter fluid component to move to the inner region of the helical flowpath; and collecting the lighter fluid component in the central conduit.
 5. The method according to claim 4, wherein the helical flowpath is such that the critical Reynolds number of the fluid flow is elevated, the fluid stream flowing at a Reynolds number below the elevated critical number.
 6. A method for separating a multiphase fluid stream including a heavier fluid component and a lighter fluid component, the method comprising: causing the fluid to flow along a first helical flowpath, the first helical flowpath having a first pitch, the first helical flowpath being sufficiently long to establish a stabilised rotating fluid flow pattern for the stream; causing the uniform rotating fluid to flow along a second helical flowpath, the second helical flowpath having a second pitch, wherein the second pitch is greater than the first pitch; and removing the lighter fluid from a radially inner region of the second helical flowpath.
 7. The method according to claim 6, wherein the multiphase fluid stream includes at least one selected from the group consisting of a gaseous phase and a liquid phase.
 8. The method according to claim 6, wherein the fluid stream comprises an aqueous phase and a hydrocarbon phase.
 9. The method according to claim 6, wherein the fluid stream includes an aqueous phase including water produced from a subterranean well and a hydrocarbon phase including oil produced from the well.
 10. The method according to claim 6, wherein the first and second helical flowpaths are formed such as to elevate the critical Reynolds number of the fluid stream.
 11. The method according to claim 6, wherein the fluid flow is stabilised in a flow regime having a Reynolds number below the critical Reynolds number.
 12. The method according to claim 11, wherein the fluid flow is stabilised in a transient flow regime.
 13. The method according to claim 6, wherein the pitch of the first helical flowpath remains constant throughout its length.
 14. The method according to claim 6, wherein the pitch of the second helical flowpath increases in the direction of flow along the second flowpath.
 15. The method according to claim 14, wherein the pitch increases in a stepwise or a continuous manner.
 16. The method according to claim 14, wherein the pitch of the second helical flowpath is increased along substantially the entire length of the second helical flowpath.
 17. The method according to claim 14, wherein the pitch increases up to 5% for each turn of the second helical flowpath around the longitudinal axis of the flowpath.
 18. The method according to claim 6, further comprising establishing a vortex separation region, in which a vortex is established into which the remaining lighter fluid fractions are caused to migrate.
 19. The method according to claim 18, wherein rotational velocity of the fluid is caused to increase, in order to establish the vortex.
 20. The method according to claim 18, further including increasing the rotational velocity of the fluid to establish the vortex, the increase in rotational velocity provided by causing the fluid to flow along a third helical flowpath.
 21. The method according to claim 20, wherein the cross-sectional area of the third helical flowpath is decreased along the length of the flowpath.
 22. The method according to claim 21, wherein the decrease in the cross-sectional area occurs in a continuous or a step-wise manner.
 23. The method according to claim 21, wherein the decrease in the cross-sectional area occurs along substantially the entire length of the third helical flowpath.
 24. The method according to claim 18, wherein the Reynolds number of the fluid flow is at or below the critical Reynolds number.
 25. The method according to claim 18, further including stabilising the vortex.
 26. The method according to claim 6, further including introducing the fluid stream into a fluid-fluid settling region, in which the lighter fluid components are separated from the heavier fluid components under the action of gravity.
 27. The method according to claim 26, further including causing the fluid stream to rotate in the fluid settling region upon exiting the vortex separation region.
 28. The method according to claim 27, further including centralising the rotational flow of the fluid stream within the fluid-fluid settling region.
 29. The method according to claim 28, wherein the cross-sectional area of the fluid flow path in the fluid-fluid settling region is reduced in the direction of flow.
 30. The method according to claim 26, wherein fluid richer in the lighter fluid component is removed from the lower central region of the fluid-fluid settling region and passed to the upper central region of the fluid-fluid settling region.
 31. The method according to claim 26, wherein the separation of the lighter fluid component from the heavier fluid components is enhanced by the addition of additives active in inducing droplet coalescence of the lighter fluid component.
 32. The method according to claim 6, further including causing the fluid stream to enter a fluid removal region, in which a fluid stream consisting essentially of the heavier fluid component is removed.
 33. The method according to claim 32, wherein the fluid stream is caused to rotate in the fluid removal zone, the fluid stream being removed from the central region of the fluid removal zone.
 34. The method according to claim 6, further including a sediment collection region, from which sediment is periodically or continuously removed.
 35. The method of claim 6, further including: providing a plurality of separation assemblies for carrying out the method recited in claim 6 and operable to accommodate different fluid flowrates; and selecting one or more separation assemblies for carrying out the method according to the volume flowrate of the multiphase fluid stream.
 36. The method according to claim 29, further including providing a finishing assembly, wherein each of the plurality of separation assemblies is connected at its outlet to the finishing assembly.
 37. An apparatus for separating a multiphase fluid stream including a heavier fluid component and a lighter fluid component, the apparatus including: a helical flowpath having a fluid inlet, a first outlet for a heavier fluid component and a second outlet for a lighter fluid component, the helical flowpath being formed such that the critical Reynolds number of the fluid stream flowing along the helical flowpath is elevated.
 38. An apparatus according to claim 37, wherein the second outlet for the lighter fluid component is disposed axially centrally of the helical path.
 39. A method for starting up a helical separation system for operation in separating a multiphase fluid stream comprising a heavier fluid component and a lighter fluid component, the method comprising: feeding to the helical separation system a first fluid stream consisting essentially of the heaver fluid component; and when the fluid velocity within the helical separation system has reached the minimum operating velocity for the multiphase fluid stream, replacing over a period of time the first fluid stream with the multiphase fluid stream to be separated.
 40. A method shutting down a helical separation system from normal operation in which a multiphase fluid stream comprising a heavier fluid component and a lighter fluid component is being fed to the helical separation system, the method comprising: introducing a first fluid stream consisting essentially of the heavier fluid component into the multiphase fluid stream feed to over time to replace the multiphase fluid stream; and when the fluid feed consists of the first fluid stream, reducing the fluid feed flowrate to zero.
 41. The method of claim 40, wherein the helical separation system is left full of the first fluid after the fluid feed flowrate has been reduced to zero. 